Why is Blender 2.8.1 constantly crashing when I select CUDA and Blender is not even in rendered view?
If you open the default blender file and it crashes on Cycles, you know it's probably not a corrupt blend file unless you saved user defaults with some setup that it doesn't like. Blender 2.82a is out with bug fixes and new features so I highly recommend getting that instead of using 2.81. Using this new version should also reset you default blend file in case you saved user defaults with corrupt settings.

Even a simple shutdown and reboot of your machine can help. You can also try unplugging unnecessary peripherals and re-seating the connections of any peripherals that are questionable (you mentioned you're using an external GPU). 

If that doesn't work then getting the most recent everything (software driver updates, firmware updates, system software updates, and 3d software updates as mentioned before) could help. Best wishes!